28psi for me at Sebring in Florida heat

 

Would love to see how the new A7 stacks in this list. NASA is calling it a 22 point tire. I heard it has more grip than the A6.

 

22 points? That'll bump you up an entire class just on it's own, and then some.

Wonder where it'll land officially?

 

Starting pressure depends on how hot they're going to get and how much water is in the air inside the tire. Now that I've started mounting my own tires I noticed that if I dont wide the tire lube off the inside of the wheel and tire before I seat the bead that I get huge pressure rises with heat because of all the water.

 

Wow, I thought the BFG R1-s getting 15 points was a bit excessive. 22 points? NASA's very subtle way of saying "Don't run this tire!"

What's the Hoosier R7 get? 13? 15?

 

It's completely car dependent, but I run 37psi hot on my setup for the 225s with...

15x9 wheels
~2350# w/ driver
-2.3 front, -1.9 rear

To Leafy's point about water, there is a way to get most of the moisture out that I heard from a local guy. I can't remember it off of the top of my head, but I'm pretty lazy when it comes to setting pressures for the day since I'm pretty slow.

My routine is to set the tires all to the same pressure at the beginning of the morning, do my first session, and bleed off pressure until they're all the same hot. I had about 200 street miles on my Rivals (including the 90 mile drive up to VIR) before my first DE this year, and the water vapor had some nasty spikes on a few of the tires. I think it was something like 42-40-39-44, bled it down to 37psi after hopping out of the car and was going to adjust if the car felt loose later on the day.

 

Not saying you're wrong. You saw what you saw. But some guy ran a pretty well controlled test of pressure build-up vs heat on the internet, and I think his results mirrored the perfect gas law. He was trying to debunk the nitrogen craze which he thought was a $$ scam. His test procedure looked valid to me and did include moisture as a factor.

 

Now I want to know how I got a set of 275s to go from 30psi to 40 just from heat and then back up to 35psi after being bled down. And then the next weekend when they were pumped back up to 30 cold they only gained 3psi getting up to the same operating temp.

 

I can tell you something from doing simulations of air bladders(not a tire, but still valid), the absolute humidity of the air will VERY much affect the rate of expansion and pressure increase when temperature fluctuates. In one simulation, dry air was compared to N2, there was very little difference. When doing simulations with some humidity, it was pretty amazing how large the effect was. Keep in mind, air coming out of an air compressor is at 100% relative humidity, for the most part.

Just for reference.

 

I have 225 Rivals on 9 in wheels. The car w/driver is around 2,500 (OK I'm on a diet). I start cold at 26. Have you checked out the Grassroots tests on the Rivals?

 

Sounds like a good water separator like the body guys use for paint would go a long way to help PSI gain without the expensive Nitrogen gas.

 

Hmm, I ran them too high before then.

 

You mean this?

https://grassrootsmotorsports.com/fo...w/63368/page1/

I did check it out before switching from R888's to the Rivals. The "pixie dust, rainbows and unicorn farts" was definitely a positive factor. I think I'll start at 30psi and see where the pyrometer leads me. Or 28. Or 29.

Two more questions...

- A comment on the review mentioned dropping pressures to get more temp into the tires (if necessary). Anybody with experience on this?

- Has anyone tried bumping up the cold pressure in the rain on the Rivals? Towards crowning helps clear water and reduces the contact patch helping get some heat into the tires. I did an A/B test on the R888's during the M@MRLS 2012 April monsoon. One session at 34psi cold - grip was OK but tires got no heat. The next (still bucketing down) at 40psi - noticeably better grip and the tire centers were steaming.

 

-Generally, you don't have any problems getting heat into street tires. I would find a pressure that gives you the most grip, but doesn't get the tires greasy before the end of your session/race. That may not be easy especially with street tires as they tend to get greasy quickly, especially if you overdrive the car. I find understeer really kills it for me, but that maybe because its not so easy for us to spin the tires while tracking out (Naturally aspirated here).
tl;dr : Find a hot pressure that doesn't get your tires greasy, avoid understeer if they get greasy anyway.

-Bumping pressure works in the rain for most tires.

 

I dump pressure in the rain for more grip.

 

let me clarify, I generally increase pressure in the rain to get more heat in the tires (also helps a bit with hydro-planing). I don't know if that leads to more grip, I have maybe 3-4 weekends in the rain.

decreasing pressure ~ more contact patch, so I am sure there is a sweet spot there.

Probably depends on the amount of rain/puddling on the track btw.

 

FWIW, industrial grade nitrogen is cheap. We bought a 302cu/ft tank several years ago. Fills 40-60 tires. Cost $26 or refills and they deliver.

edit: and on the current topic. We run very high pressures in full wet conditions.

 

Availability and available gear (regulators etc) control suitability, but scuba air is just as dry as tech Nitrogen.
My small 0.85L 200bar bottle was capable to fill 4 empty 205 to 2 bar.
Using a 3L 300 bar bottle now, lasts a whole season for me and my buddies.

 

I just picked up a set of RA1's since they were less expensive than NT01's. Based on what I've read the RA1 and NT01 are pretty similar. The tread compound on the RA1 changed at some point. I found the RA1 to be very easy to drive. I found them more confidence inspiring than the Yoko AD08's I had previously on the car.

 

No, that's a forum post. Andy Hollis did tire testing articles, IIRC, two months in a row. The first one briefly mention the Rivals and the second went into more depth. I'm thinking it was 8 or 9 months ago. I'd have to dig through my 'paper' magazines to find them. The guy who helped him with the testing AutoXs and runs NASA TTE locally. He suggested 25 cold for my 225s on 9s.